The central data bank of the Rochester Program Project (RPP) consists of a file of medical diagnoses and surgical procedures covering the population of Rochester and Olmsted County, MN, for a period of more than 40 years. These data have been assembled from records at Mayo Clinic and other medical care centers and are considered a national resource for epidemiological studies. The medical records upon which this resource depends cover essentially all local patient-physician contacts and include records for home, outpatient and hospital visits, and autopsy data. This resource is maintained at the Mayo Clinic but is available to visiting scientists, students from other universities, and health agencies. More than 100 epidemiological studies using this data bank have been completed. These include studies of incidence and outcome as well as case-control comparisons and prospective studies. Recently completed studies include those on pituitary adenomas, carcinoma of the lung, kidneys, ureter, gallbladder and extrahepatic biliary ducts, primary brain tumors, leukemia, rheumatoid arthritis, diabetes mellitus, cerebrovascular disease, stroke, and computed axial tomography brain scan. Underway at the present time are several descriptive epidemiologic studies on ischemic heart disease, retinal detachment, prostate cancer, multiple myeloma, alopecia areata, head trauma, renal stones, and sarcoidosis; case-control studies of leukemia, estrogen use in women with ischemic heart disease, rheumatoid arthritis, fracture risk in diabetics, progestin use in pregnancy, and studies of the use of the CAT scan and total body scans.